Is it possible to replace only 1 piston on a 4 cylinder 115?

[Billy Ray]

I understand that compression is very important and if I have to bore out a cylinder and install a bigger piston the compression will be much higher than the other three cylinders.  If that is the case than I will have to do that.  This question is slightly premature because I have only taken the head off of the motor to look at the pistons.  Believe it or not, I have melted the top of one piston.  So, my motor is only running on three cylinders.  I do not know if I have to bore out the cylinder yet.

     My question is:  If the cylinder's ok....can I just replace the piston and rings?  It would obviously be much cheaper.  I know that if I'm going to go through all of the trouble of taking the motor apart that I should go ahead and replace all four pistons and cylinders but I do not want to rebuild the motor.  Money is obviously an issue.  Thanks for any and all help.

[GoneFission]

You can replace 1 piston and rings if the cylinder bore is OK.  But - here is the important part - you need to determine WHY the piston was damaged in the first place and fix that!  It's likely to be a partially clogged carburetor main jet that caused one cylinder to run lean, overheating and damaging the piston.  Sometimes a sticky reed valve can also cause a lean condition.  You may find all the pistons have some damage - that is likely caused by the ignition timing set too far advanced or having the carbs jetted wrong.  

Assuming the cylinder bore is not chrome-plated,  you will need to lightly hone the cylinder bore to ensure the new rings will seat and fit the rings to the correct end gap.  I like the flat stone hone better than the ball type, especially for two stroke engines.  You want a nice criss/cross pattern to the honing to help the rings seat.  

Proper piston ring end gap is critical due to differences in manufacturing tolerances. You can check the gap by putting the ring in the cylinder and then using the top of the piston to push the ring into the bore a bit, then pull the piston out and use a feeler gauge to check the end gap.  You can use a regular file to adjust ring end gap, but be careful to keep the edges flat and straight.  Some "shadetree" types don't check end gap, and the result is a broken ring soon after the motor is placed in service.  You may get lucky and have a good end gap out of the box, but check it!    

Install the rings on the piston, with gaps on opposite sides.  Use a ring compressor and push the piston into the cylinder bore.  Make sure the bearings are clean - you don't want dirt or metal here!  Put some pre-lube compound or STP on the rod/crankshaft bearings so they are lubricated for the first start-up.  Clean and dry the rod bolt threads and install the rod caps.  Use some Locktite Blue on the rod nuts and torque them in 2 or 3 stages.  Let them sit an hour and then final torque again.  

Clean the head bolts carefully (or buy new ones if they are badly corroded) and use some non-permanent thread sealer on the threads.  Make sure you get the right head gasket for your model and torque the head bolts carefully in 3 stages, including a retorque after the engine warms up the first time.  

Hope this helps - see ya on the water!

[Billy Ray]

Very informative.  Thanks !!

[Boatdood]

Capn John gave you some good advice. I'd just like to add a few things. Increasing the bore of a cylinder does not increase compression. It does add displacement but the added displacement is minimal in a short stroke engine such as your outboard. In 1977 OMC came out with a nice TRW forged piston in their V-4 engines. The problem was that they had the piston ring all the way at the top of the piston. That design worked great to make the rings seal against the cylinder wall but it led to premature ring failures all too often. To further complicate matters they also drilled too deep on the piston ring retaining pins. This allowed the pins to sink too low into the piston and the rings could then spin on the piston. Once this started it wasn't long before the end of the ring ended up in a port and broke the ring. So what they had was a physically superior piston with poor design resulting in premature catastrophic engine failure. I told them what the problems were but they pretty much ignored me, much to my pleasure as I proceeded to make a bunch of money fixing their mistakes. Sometime in '79 they came out with the same piston but with the rings dropped down by about one quarter of an inch from the top. This lowered the compression a bit but it did result in much better longevity of the piston and the engine. If your engine has the low ring pistons you're in good shape. If you have the high ring pistons you've been living on borrowed time for many years already. Either way, replacing only one piston should be fine, as long as the rest of the stuff in the engine checks out OK.